I echo everything already said! Pick a comfortable pace. Stop and rest and drink water!! Save the frostier beverages for relaxing in the evenings. Don't hesitate to go back to your resort mid-day to rest.
There are some great "rides" like the Magic Kingdom train that are fun AND restful. You can ride all the way around the park, see it from a different perspective, and rest. Same for the monorails. We ride them just for the ride, to Epcot or around to the resorts at MK. The Kilimanjaro Safari at AK is also a nice ride. Check the line, though. It can be a long wait if standing is an issue. Each park has that kind of 'ride' that isn't high stress or motion.
And spend some time at Downtown Disney. Shopping and for us, it is a great place to relax! We have grabbed lunch, found a waterside table or bench, and just soaked up the vacation. Crowds are lighter, in general, pace is like a Mall, and it is a great break from the parks. Good food, too!
Did you notice we are all about the food here?
You're gonna have a Great Time!
__________________
If more of us valued food and cheer and song above hoarded gold, it would be a merrier world.
J. R. R. Tolkien